Color flat screens of the transmissive type, such as flat liquid crystal screens, used in avionics and in particular in the military field, comprise a multitude of pixels whose transparency is controlled by an electrical voltage. Each pixel is controlled separately with a view of forming an image on the screen. In the case of color images, each pixel is usually composed of a mosaic of three sub-pixels of different color and independently controlled. The final colors of the pixels that result therefrom, following an additive combination of light within the observer's eyes, are obtained by a particular combination of the control voltages for each sub-pixel.
In order to allow them to display, flat color screens of the transmissive type must be backlit by means of an auxiliary light source called a light box. For correct color rendering, this light source must illuminate the screen with white light as uniformly as possible.
Moreover, the screen must be able to be consulted under very varied ambient brightness conditions, for example during the day in full sunshine or when cloudy, and at night with low or zero illumination. These constraints require the use of a light source for backlighting with adjustable brightness over a wide range of light intensities, possibly with a ratio of up to 50, while maintaining a constant color temperature or emission spectrum in order to prevent deterioration of the colors of the backlit screen.
To obtain this large dynamic brightness range, light boxes for backlighting a transmissive flat color screen of the prior art comprise two stages of light sources placed behind a diffusing screen. The high brightness illumination for daytime mode is achieved using a first stage of light sources formed from a row of fluorescent tubes placed as a first curtain behind the diffusing screen. The low brightness illumination for one or more night-time modes is achieved by means of a second stage of light sources formed from a lower number of fluorescent lamps placed either as a second curtain, behind the first lamps, or along the sides of the light box.
Light boxes based on the use of fluorescent tubes have many drawbacks. The number of tubes used in the light box, because of their size, remains small, which has the drawback in the event of one of the tubes failing, despite the presence of the diffuser, of making the backlighting for the transmission display screen inhomogeneous. Furthermore, since the fluorescent tubes used for low lighting (night-time mode) are appreciably fewer than those used for high brightness lighting (daytime mode), they form an even more inhomogeneous light rail, with which it is more difficult to obtain uniform illumination of the screen. For this reason, the tubes for low lighting are furthest away from the diffusing screen and are always used in indirect lighting. The fluorescent tubes used for low lighting are placed as a second curtain, the tubes used for high brightness lighting, placed as a first curtain, serve as a mask preventing, by their presence in the path of the light rays from the tubes placed as the second curtain, direct illumination of the diffusing screen. When the second row of fluorescent tubes is placed along the sides of the light box, the tubes are coupled to a cavity in the light box via a particular optical waveguide. These arrangements of the fluorescent tubes result in light boxes of large size and high cost. Furthermore, the light boxes using fluorescent tubes or lamps pose problems of extracting the heat. This is because, as it is sought to produce a light box of smaller volume, the heat produced by the tubes builds up in a small volume; this heat is usually removed by thermal conduction through the rear face of the light box, which is contradictory to the position, near the diffuser screen, of the tubes of the first curtain that give the strongest illumination and therefore produce most of the heat in the box.
Furthermore, the use of fluorescent tubes, with a broad spectrum extending into the infrared, for providing low brightness illumination in night-time mode, poses problems of dazzling night vision amplifiers, especially infrared night vision goggles. An infrared filter for the tubes in night-time mode is then necessary, which increases the complexity of the light box, reduces the light intensity of the tubes and exacerbates the problem of heat dissipation due to the heat output by the tubes.
The subject of the present invention is a backlighting device, having several brightness levels, that provides improved solutions to the abovementioned problems and has a structure which is both simple and very compact. Therefore the invention proposes a backlighting device, having several brightness levels, for a transmission display screen, comprising light sources placed in a case between a translucent front wall formed by a diffuser screen and a rear wall, the light sources being selected according to the desired brightness level, characterized in that the light sources are mosaics of light-emitting diodes that are placed immediately behind the diffuser screen with, for the highest brightness level, a first mosaic of light-emitting diodes that provides daytime illumination and with, for the lowest brightness level, a second mosaic of light sources that supplies night-time illumination.
In a first embodiment of the backlighting device according to the invention, the first mosaic and the second mosaic of light-emitting diodes lie in the same plane, the light-emitting diodes being wired to the same printed circuit.
The use of light-emitting diodes provides great flexibility in adapting the intensity of illumination according to the ambient conditions. This is because, whether one is in the daytime mode or in the night-time mode by the respective use of the first or the second mosaic of light-emitting diodes, the light intensity may be easily adjusted by controlling the supply current to the light-emitting diodes. Thus, whether in daytime mode or in night-time mode, the light intensity may be matched perfectly to the particular ambient conditions. For example, in daytime mode, the light intensity of the first mosaic may be adjusted so as to emit light of maximum intensity when the backlit screen is consulted in full sunlight and reduced if the ambient light drops because of the presence of clouds or because it fails with the onset of dusk.
For this purpose, each mosaic is formed from a defined number of arrays of light-emitting diodes. The light intensity of the light-emitting diodes of an array is adjusted by controlling the current through the diodes by a semiconductor. The diodes of an array of light-emitting diodes in series with a semiconductor are preferably dispersed over the mosaics. This is because, should the transistor or one of the diodes of the array fail, the resulting change in brightness will be diffuse and not concentrated in one area of the light box, as is the case with fluorescent-tube light boxes of the prior art.
When the light box is used in night-time mode with equipment that is very sensitive to infrared radiation, a very low level of radiation emitted by the diodes may be sufficient to saturate such equipment. Advantageously, the light box includes an infrared filter between the diffuser and the mosaics of diodes that further reduces the low amount of infrared radiation able to be produced by the second mosaic of diodes.